Because of their high energy density, alkaline battery systems have great potential for replacing the more conventional lead-acid battery system in a number of terrestrial applications. However, extending the cyclic life of such batteries beyond that presently attainable and reducing the cost of all the components are required criterias which must be met to make the alkaline battery system an effective energy source.
One of the recognized key components in attaining an extended battery life and efficiency is the battery separator. In alkaline battery systems, such as a nickel-cadmium battery, where a dendristatic separator diaphragm is not required, the separator can be a porous diaphragm suitably located between the positive and negative plates of the system so as to (1) provide a separation between electrodes of opposite charge, (2) provide an electrolyte reservoir, (3) provide uniform electrolyte distribution across electrode surfaces to permit uniform current density and, (4) allow space for electrode expansion during use. In order to achieve these results, the resultant diaphragm must be capable of exhibiting a high degree of absorption or wicking, and be sufficiently porous to carry and evenly distribute the electrolyte of the battery system.
It is also desirable to have a separator diaphragm which is very thin, such as less than about 10 mils, in order to minimize the amount of electrolyte required and, thereby, maximize the energy density of the resultant system. It is further desirable that the separator be of a material which is sufficiently flexible and thermoplastic to permit formation of an envelope or pocket configuration around the electrodes to further add in maintaining separation.
Conventional lead-acid and secondary alkaline battery systems have certain components, such as electrodes, electrolytes, separators, etc., which, though commonly named, are distinctly different entities having different functions, and are required to have different physical and chemical properties. It is easily recognized that the electrodes of a lead-acid battery system are distinctly different from the electrodes used in a secondary alkaline battery system, such as a nickel-zinc or nickel-cadmium alkaline battery system. Similarly, separators used in a lead-acid system are distinctly different from those used in a secondary alkaline battery system. The lead-acid battery separator is a material positioned between electrode plates of opposite polarity merely to insure maintenance of a separation. Any material which is superimposed between adjacent surfaces of the plates to retain the desired separation is satisfactory. Normally, these separators are produced of materials which are capable of being formed into sheets of (1) substantial thickness or with a matte surface to aid in the separation of the plates, (2) substantial porosity to readily permit the electrolyte to pass through, and (3) must be chemically inert to acidic electrolyte. Separators used in certain alkaline battery systems, such as nickel-zinc battery, are not only required to aid in separating the plates of opposite polarity, but must also function as a dendristatic diaphragm. The separator required in such alkaline battery systems where dendritic growth occurs, must, therefore, have very small pore size in order to inhibit the growth of dendrites therethrough, must be very thin to minimize electrical resistance, and must be of a material which is chemically inert to alkaline electrolyte while allowing electrolyte passage therethrough.
Separators useful in alkaline battery systems in which dendrite growth is not found, such as nickel-cadmium systems, have different structural requirements. Such separators should be very thin, chemically inert to alkaline electrolyte and, at the same time have a high degree of wicking properties to maintain electrolyte over the entire surface of the electrodes. High wicking capability, such as 5 cm/24 hours as determined by standard technique are desired.
Battery separators which are used today in alkaline battery systems are commonly composed of polypropylene, polyamide, or nylon non-woven sheets. These separators suffer from insufficient wicking and/or lack the necessary chemical and/or oxidation resistance in an alkaline environment to effectively aid in enhancing the battery system. The development of alkaline secondary batteries, particularly nickel-cadmium, has been hampered by the absence of suitable separators.
It is an object of the present invention to provide a fibrous, filled battery separator suitable for use in alkaline battery systems, especially systems which do not require dendristatic separator membranes.
It is another object of the present invention to provide a battery separator that can be prepared on a conventional paper machine.
It is still another object of the present invention to provide a battery separator of a thinness of not greater than about 10 mils which can be prepared on a paper machine and exhibits a tensile strength of greater than 200 lbs/in.sup.2 in the machine direction, and sufficient flexibility to be formed around the electrode plate.